This invention relates to a display device of the kind comprising a frame having a length and a width and, mounted on the frame, at least one circuit board which is capable of being flexed and which is provided with electric circuit means for magnetically controlling the positions of movable electromagnetically controllable display elements having at least two display surfaces respectively of different appearance, the display elements being mounted in a lengthwise and widthwise extending array on the frame. In particular, the display device is intended to be positioned against a transparent fronting screen, e.g. a fixed windscreen or display screen of a bus or other vehicle.
In a known display screen of the kind referred to, the main purpose of the frame is to provide a rigid support for the at least one circuit board so that the latter are not subjected to varying stresses prior to the display device being mounted in its final mounted position. Thus, in such known display devices, the frame is deliberately constructed as a rigid structure carrying the at least one circuit board. Normally the rigid frame supports the at least one circuit board in a common plane. However, if the display device is to be positioned behind a curved screen, e.g. a curved windscreen of a bus, the frame may be deliberately curved to support a number of circuit boards in a curvilinear configuration.
It is also known from U.S. Pat. No. 3,947,984 to deliberately flex a circuit board and to mount the circuit board on a frame in its flexed condition. However, the purpose of flexing the circuit board is to curve the board so as to create a channel effect which resists bending or distortion about lines transverse to the direction about which curvature of the circuit board takes place.
Conventionally different models of buses, although possibly having approximately the same width, have differently configured windscreens. The windscreens may be planar but, more usually, are curved, the curvature of each windscreen normally being greater at its two sides than in the central portion. Although it is possible to mount a standard display device having its array of display elements arranged in a common plane behind a number of differently configured windscreens, this is not ideal since the front of the display device will normally be spaced from the windscreen at least along part of the length of the display device. If the front of the display device does not closely conform to the windscreen the visibility from the front of the display device will be affected. Although this problem can be overcome by curving the frame as previously described to the configuration of the windscreen with which the display device is to be used, such a solution is not ideal since a number of different display device configurations have to be manufactured for use with the different windscreen configurations.